Wind turbines exploit wind energy by converting the wind energy to electricity for distribution to users. Wind turbines are typically sited at isolated locations where the average wind speed is sufficient to generate economically-feasible electricity or where the grid may be regarded as relatively “weak” due to the few generating plants and long distances between plants. A “weak” system is characterized by a lack of robustness, a relatively low short circuit strength, e.g., less than about 10 kA and/or a variations in voltage at different points on the system.
A wind turbine comprises a nacelle enclosing a generator and other power conditioning equipment. Typically three turbine blades, extending from the nacelle, have an axis of rotation substantially parallel to the surface of the ground on which the wind turbine is mounted. The wind-driven blades drive the generator within the nacelle to generate electricity.
The nacelle is disposed at the top of a tower, which is conventionally circular in cross-section and may be as tall as about 80 meters. Current wind turbine designs include towers about 99 meters high. Since the tower height is typically measured from a horizontal centerline of the nacelle, the actual tower height is slightly less than the referenced figures.
Each tower comprises multiple sections. An 80 meter tower comprises three sections; a 99 meter tower has four or five sections. The length of each section depends on the weight of the tower, as weight is generally more restrictive for transporting the tower than tower length, since each tower is broken down into sections.
Since the speed of prevailing winds tends to increase with increasing altitude, the tower length is selected to take advantage of this condition, giving due consideration to the structural supports required with increasing tower height and additional wind loads. The nacelle is rotatable about the vertical axis of the tower to maintain proper orientation of the turbine blades relative to a wind direction.
Wind turbine locations are typically a significant distance from the manufacturing location of the wind turbine components, including in particular the supporting tower. The components and the tower must therefore be transported from the manufacturing site to the installation site where the turbine is assembled/erected and placed into operation. Various modes of transportation may be utilized during transit, including barges, railcars, vehicular trucks, etc.
Generally, wind turbine towers have a taper from a support base to a support top, as this structural feature provides sufficient strength while utilizing minimal material and limiting fabrication costs.
Transportation of the support tower is particularly problematic due to its length, weight (typically about 125,000 pounds), taper profile, and circular cross-section. Since the tower lacks a flat surface, it is difficult to stabilize the tower during transportation. Support fixtures must therefore be employed to provide the necessary stabilization and cushioning during transportation. As alluded to above, the towers are shipped in sections (and assembled at the installation site) to minimize these transportation problems.
One technique for increasing the power-generating capacity of wind turbines increases the blade length. This factor also necessitates the use of longer (i.e., taller) support towers, which further exacerbates the transportation problems.